1. Field of the Invention
The present invention relates to an optical frequency shift keying modulator and the like.
2. Description of the Related Art
An optical frequency shift keying (optical FSK) is a technology which applies modulation to a frequency of light and transmits variations in the frequency as a signal. An FSK signal generally carries no information on its amplitude, so that it has a feature of being subject to a level fluctuation or noise.
An FSK system using a digital signal has been already known (see e.g. Japanese patent application laid-open No.11-17746). However, this technology is related to shifting the frequency of the digital signal, so that the frequency of light is not shifted.
As a system for shifting a frequency of light, there is an optical FSK system. In a conventional optical FSK system, a laser emission wavelength itself is changed by varying an electric current provided to a tunable laser light source. On a receiver's side, received laser light is separated into components per wavelength by a branching filter, converted into electric signals and intensities thereof are measured by a photodetector, and a difference thereof is obtained by a subtracter. However, in an optical FSK system of such a method, a laser intensity changes as the laser wavelength is changed, so that there is a problem that this must be compensated. Moreover, there is a problem that such a system cannot accommodate to a high-speed operation.
Also, as an apparatus capable of changing a frequency of inputted light, an optical single sideband modulator (optical SSB modulator) is known. FIG. 1 is a block diagram showing a basic arrangement of an optical SSB modulator. As shown in FIG. 1, an optical SSB modulator 1 is provided with: a first sub Mach-Zehnder waveguide (MZA) 2; a second sub Mach-Zehnder waveguide (MZB) 3; a main Mach-Zehnder waveguide (MZC) 4 including the MZA and MZB; a first electrode (DCA electrode) 5 controlling a bias voltage between two arms composing the MZA, thereby controlling a phase of light propagating in the two arms of the MZA; a second electrode (DCB electrode) 6 controlling a bias voltage between two arms composing the MZB, thereby controlling a phase of light propagating in the two arms of the MZB; a first RF electrode (RFA electrode) 7 inputting a radio frequency (RF) signal to the two arms composing the MZA; a second RF electrode (RFB electrode) 8 inputting the RF signal to the two arms composing the MZB; and a direct-current or low-frequency electrode (DCC electrode) 9 controlling the bias voltages of the MZA and MZB, thereby controlling a phase of light propagating in the MZA and MZB. It is to be noted that “low frequency” in the low-frequency electrode denotes a frequency of e.g. 0 Hz-500 MHz.
Namely, in the optical SSB modulator, the direct-current or low-frequency electrode (DCC electrode) is used for controlling the phase of light propagating in the MZA and MZB. It is to be noted that the optical SSB modulator is reported in detail in “Optical SSB modulator using integrated LN modulator” (Shimotsu et al., Technical Report of IEICE, OEIC. OPE2000-37, LQE2000-31(2000-07), 29-34, 2000).
Although a frequency of the output light can be changed by the optical SSB modulator, there is a limit to a response speed of a control circuit for changing the frequency, so that a speed of the frequency change by the optical SSB modulator has a limit of approximately 10 ns. Originally, the optical SSB modulator is not intended for shifting the frequency of the output light so as to use the shifted frequency as information. Therefore, there is a problem that the optical SSB modulator is not exactly suitable for an optical FSK modulator. Japanese patent application laid-open No.11-17746